The invention relates to a method to measure and correct or adjust the sound signal presented to the eardrum by means of a hearing aid in the operational position, including at least one microphone, at least one digital signal processing system comprising at least one digital signal processor for transforming the incoming sound signal into a transformed signal in conformity with the desired transformation function, and at least one receiver and a power supply, and having at least one sensing means for sensing the signal appearing in front of the eardrum, and at least one comparison means.
Measurements and corrections for linear or nonlinear distortions in hearing aids are known from the prior art, particularly from German Publication DE 28 085 16, which discloses a hearing aid, which in addition to the receiver uses a measurement microphone or probe microphone, which could be separate from the receiver or incorporated or integrated into the receiver. This microphone picks up the sound environment in the ear canal in front of the eardrum and is used for the compensation of linear and/or nonlinear distortions of the signal.
The instantaneous analog values of the output signal of the probe microphone are applied at one input of a differential amplifier, the second input of which receives the undistorted output signal of a preamplifier of the hearing aid. The output signal of the differential amplifier is then applied as a correction voltage which is added to the input signal of the output amplifier, resulting in a corrected output signal from the receiver.
Thus, the probe microphone and the differential amplifier are part of a feedback loop for correcting distortions of the output signals of a hearing aid.
However, this known system can not adapt itself in real time to instantaneous variations of the entire electroacoustic system, comprising of the ear and the hearing aid, preferably a programmable or program controlled digital hearing aid system.
In U.S. Pat. No. 4,596,902, a hearing aid is disclosed having a feedback microphone located in the ear canal when the hearing aid is in use. The feedback microphone monitors actual sound pressure levels in the ear canal, and the hearing aid adjusts individual gains in a plurality of frequency bands in response to a comparison of the monitored sound pressure in the ear canal and in the frequency band in question with a respective predetermined value so that the sound pressure level is kept below a loudness discomfort level in each frequency band.
In Widin G. P.: “The meaning of digital technology”, Hearing Instruments, vol. 38, No. 11,1 November 1987, various types of use of digital signal processing in hearing aids are discussed in general. The discussion is divided into discussions of use of computers in hearing instrument fitting, use of digital circuitry to control analogue electronics, use of digital signal processing to replace analogue circuits to accomplish standard hearing instrument functions, and use of digital techniques to produce new kinds of signal processing, such as noise suppression.
CH 624 524 A discloses a hearing aid with a microphone, an amplifier and a loudspeaker. The hearing aid further comprises a feedback microphone for monitoring sound emitted by the loudspeaker and generating an output signal that is fed back into the amplifier for correction of the output generated by the hearing aid.
It is an object of the present invention to create and develop a novel method for an instantaneous measurement and correction or adaption of the sound environment in front of the eardrum, even including occlusion effects and other foreign signals or sounds influencing the sound field in front of the eardrum, to a desired sound signal.
A model function of this type may be developed and one may even be able to predict or anticipate changes in the sound environment in front of the eardrum by such a method.